2-[2-Methyl-prop-1-en-1-yl]-4-methyl-tetrahydropyran, a terpenic ether of formula ##STR4## better known by its common designation of "rose oxide", is a particularly valuable ingredient for perfumery.
Ever since its discovery in 1960 [cf. Swiss Pat. No. 395,406], numerous publications relative to its preparation have appeared in scientific journals and in the patent literature.
Most of the known methods include the use of acyclic intermediates or the introduction of the 2-isobutenyl substituent in the preliminary formed pyran ring. A different synthetic approach, starting from 3-methyl-but-2-en-1-al and 2-methyl-but-1-en-4-ol, was suggested by J. P. H. Tyman and B. J. Willis [cf. Tetrahedron Letters, 51, 4507 (1970)] which method can be illustrated by the following reaction pathway: ##STR5## According to the above cited authors the reduction of the methylene intermediate, or rose dehydroxide, is carried out by hydrogenation in a homogeneous phase in the presence of tin.sup.II chloride and hexachloropatinic acid. It was thus possible to obtain a mixture comprising about 91% of cis isomer and about 9% trans isomer.
The International Application No. WO 79/00509 relates to a process destined to prepare rose oxide under the form of a mixture containing eminently the cis isomer, which process makes also use of 3-methyl-but-2-en-1-al and 2-methyl-but-1-en-4-ol as starting materials but contemplates an additional isomerization step by means of an acidic agent.
The commercial interest presented by rose oxide has further stimulated the search for alternative synthetic routes. This invention provides a novel means for its preparation.